1. Field of the Invention
The present invention relates to a decompositionally treating agent and decompositionally treating method for fluorocarbons. More particularly, it is concerned with a decompositionally treating agent and decompositionally treating method that are capable of decompositionally treating fluorocarbons such as CF4 in high efficiency for a long period of time at a relatively low temperature of 1000° C. or lower, which fluorocarbons are contained in an exhaust gas exhausted from a semiconductor manufacturing process or the like.
2. Description of the Related Arts
In a semiconductor manufacturing industry, there are used fluorocarbons such as CF4, C2F6, C4F6 and C5F8 as an etching gas in a dry etching apparatus or a chamber cleaning gas in a CVD apparatus. Since these fluorocarbons are each a highly stable compound and exert great influence on the global warming, there is caused anxiety about evil influence on the environment when released in the atmosphere. Such being the case, it is preferable to recover or decompose the fluorocarbons contained in an exhaust gas exhausted from a semiconductor manufacturing process or the like, and thereafter release in the atmosphere.
The exhaust gas generated after the conventionally employed fluorocarbons such as CF4, C2F6, C4F6 and C5F8 are used as an etching gas or a chamber cleaning gas frequently contains the above-mentioned fluorocarbons and besides such sour gases as HF, F2 and SiF4, usually in addition to the basis gases including nitrogen, argon and helium. Since the concentration of these fluorocarbons contained in an exhaust gas is usually about 10 to 50000 ppm, which is relatively in a low level, attempts have been made to treat the fluorocarbons mainly by a decomposing method which is inexpensive in running cost.
As a decompositionally treating method for fluorocarbons, there have hitherto been adopted, for instance, a method in which a fluorocarbon-containing exhaust gas is introduced into the flame in a incinerator where hydrogen, methane or propane is used as a fuel to cause fluorocarbon combustion, and a method in which oxygen, air or a mixture of oxygen or air along with moisture is added to a fluorocarbon-containing exhaust gas, and the resultant mixed gas is heated and oxidized. There have also been developed a method in which a fluorine compound such as a fluorocarbon is brought into contact with molecular oxygen in the presence of alumina {refer to Japanese Patent Application Laid-Open No. 286434/1998 (Heisei 10)}, a method in which a fluorine compound is brought into contact with a decompositionally treating catalyst wherein a metal belonging to group 6A, 8 or 3B of the Periodic Table and an inorganic acid such as sulfuric acid, phosphoric acid or boric acid are supported on alumina {refer to Japanese Patent Application Laid-Open No. 165071/1999 (Heisei 11)}, a method in which a fluorine compound is passed through a catalyst bed comprising the mixture of an alumina base catalyst heated to 300 to 1000° C. with a silica-containing mixed member in the coexistence of oxygen and water {refer to Japanese Patent Application Laid-Open No. 15060/2000 (Heisei 12)} and the like methods.
However, the decompositionally treating method by means of combustion suffers from such disadvantages that a combustional state is obliged to be maintained even at standby time when the fluorocarbons are not decompositionally treated, whereby energy cost is markedly increased and besides, a large amount of carbon dioxide gas is released into the atmosphere. The decompositionally treating method by means of heating oxidation with the addition of air or oxygen suffers from such drawbacks that heating to at least 1000° C. is necessary, and a further higher temperature is needed on the decomposition of CF4, whereby the commercialization of a decompositionally treating apparatus is made difficult from the viewpoints of corrosion resistance as well as heat resistance.
The decompositionally treating method for fluorocarbons by means of alumina as the decomposing catalyst is advantageous in its capability of decomposing the same at a relatively low temperature. Nevertheless, the foregoing method suffers from such a shortcoming that the reaction of the fluorocarbon with the alumina forms aluminum fluoride on the surface of the alumina, whereby the decomposing catalyst is deactivated in a short period of time. The decompositionally treating catalyst wherein a metal and an inorganic acid or silica are added to alumina has been developed for the purpose of maintaining the catalytic activity of the catalyst for a relatively long period of time. However, in the case where the fluorocarbon as the treatment object is CF4, it is difficult to decompositionally treat the same continuously for a long time, while maintaining a decomposition rate of around 100% at a temperature of 1000° C. or lower.
In the case of decompositionally treating fluorocarbons in the coexistence of water, it is possible to enhance the decomposition rate. Nevertheless, the foregoing method suffers from such defects that the formation of hydrogen fluoride after the decompositional treatment necessitates the removal of the hydrogen fluoride with a wet cleaning unit or the like prior to releasing the exhaust gas in the atmosphere, and besides that the exhaust gas exhausted from a decompositionally treating unit makes it impossible to use a heat exchanger owing to its high temperature and corrosiveness.